In such mechanical power amplifiers, in which the power transmission ratio is produced on the one hand by the wedge surface inclined in opposition to the direction of movement of the wedge and on the other hand by the difference in length of the lever arms of the bell-crank levers, there are two types differing constructionally somewhat from one another, but which operate on the same principle and differ merely by the respective interchange of the arrangement of the abutment and secondary member. Thus, a mechanical power amplifier is known (from U.S. Pat. No. 2,335,743) in which the abutment for each bell-crank lever is a spindle mounted rigidly in the housing. A cylindrical roller is rotatably mounted on the said spindle. The short lever arm of each bell-crank lever, by means of a further spindle, engages a yoke-like secondary member common to both bell-crank levers. On displacement of a primary member provided between the long lever arms of the bell-crank levers and having two edge surfaces turned towards each other, the yoke-like secondary member is moved in opposition to the direction of movement of the primary member. Thus, in the power stroke, the secondary member is drawn into the housing in which the bell-crank levers are mounted. It is connected by a pull-rod to the movable clamping jaw of a vice with support.
In another known power amplifier (U.S. Pat. No. 2,443,775), each bell-crank lever is pivotally mounted in its middle part on a spindle rigidly mounted in the housing. The short lever arm of each bell-crank lever carries a cylindrical roller rotatable on a spindle provided on the short lever arm. The said roller acts on a secondary member which, on displacement of a primary member having two wedge surfaces, is moved in the same direction as the primary member. Each of the two secondary members acts on the movable clamping jaw of a vice.
In both known mechanical power amplifiers, therefore, two spindles are provided for each bell-crank lever, one of which spindles serves for mounting the cylindrical roller while the bell-crank lever is pivoted on the other. These spindles are subjected to considerable forces and are thus exposed to heavy wear. Since they can be loaded only to a limited extent, the known mechanical power amplifiers are either of large overall size or only relatively small clamping forces can be produced with them. In addition, owing to the said spindles, the number of necessary individual parts is increased, and therefore the production and assembly costs are also increased. Finally, however, the transmission ratio, once fixed constructionally, is either not variable at all or is variable only with very great difficulty.
The problem underlying the invention is, therefore, to provide a mechanical power amplifier, in particular for vices of the aforesaid type, which is of simpler construction, has fewer individual parts, is cheaper to make, is of small overall size, operates with little wear and in which, if necessary, the transmission ratio is easily variable.
This is achieved according to the invention in that the short lever arm on its outer side turned towards the wedge surface has a convex rolling surface with generatrices extending perpendicularly to the plane of movement of the bell-crank lever, by means of which surface the said lever arm is supported on a plane counter-surface of the secondary member or of the abutment, the said counter-surface being turned away from the wedge surface, and each bell-crank lever on its back turned away from the wedge surface has a projection approximately at the level of the rolling surface, by means of which the bell-crank lever is supported on the projection of a second bell-crank lever or on the wall of a housing.
There are no spindles in the new mechanical power amplifier. The last-mentioned is thereby simpler to produce and assemble. All the heavily loaded co-operating parts roll upon one another with line contact and free from slip, so that friction and therefore also wear are reduced to a minimum.
Thus, the rolling surfaces of the short lever arms roll on the corresponding counter-surfaces, as also the cylindrical rollers roll on the supporting surfaces of the bell-crank levers or on the oppositely situated counter-surfaces. Due to the absence of spindles, however, the overall size is substantially reduced, or in the case of equal overall size, compared with known mechanical power amplifiers of this type, a considerably greater clamping force can be produced. In addition, it is also possible in a comparatively simple manner to vary the transmission ratio of the power amplifier by shifting the cylindrical roller, with respect to the short lever arm, more or less to the outside. When it is stated here that the cylindrical roller is freely movable, this means that the cylindrical roller does not rotate on a definite fixed axis or spindle, but only on its own longitudinal axis. As the rolling movement of the cylindrical roller on the counter-surface, the said longitudinal axis is displaced.
Advantageously, the counter-surfaces should extend at right-angles to the direction of movement of the primary member, so that transmission ratios are obtained which are simple to grasp, and in addition the bell-crank levers always remain in their predetermined position.
In order that the predetermined transmission ratio of the power amplifier does not alter in the event of any unforeseen strong vibrations, which could result in shifting of the cylindrical rollers, it is expedient if the counter-surface, on which the cylindrical rollers roll, in its direction at right-angles to the direction of movement of the primary member is bounded on each side by a recess. These recesses prevent movement of the cylindrical roller beyond the recess, and by the positive abutment ensure that both rollers remain and operate at the same distance with respect to the central axis.
Another advantageous embodiment is that the supporting surface at the central part of the relaxed power amplifier is inclined with respect to the direction of movement of the primary member, and in the clamped position of the power amplifier extends approximately at right-angles to the direction of movement of the primary member. In this case, the counter-surface co-operating with the cylindrical roller should be held by spring force against the cylindrical roller and the latter against the supporting surface of the bell-crank lever for prevented unwanted movement of the cylindrical roller with respect to the surfaces co-operating with it. In addition, the spring force keeps the bell-crank levers in their correct position.
Further advantageous embodiments result from sub-claims 6 to 9.